Liquid crystal optical devices have merits such as low power consumption, small thickness or light weight, and they are widely used for many electronic devices such as cellphones, digital cameras, portable information devices or TVs. Among these, in recent years, liquid crystal optical devices are proposed, in which electric field is controlled to control alignment of liquid crystal molecules to thereby change a light-scattering state.
Further, liquid crystal optical devices (hereinafter referred to as liquid crystal/cured material composite device) such as LCPC (Liquid Crystal Polymer Composite), PDLC (Polymer Dispersed Liquid Crystal) or NCAP (Nematic Curvilinear Aligned Phase) which each comprises a composite of liquid crystal and cured material, are known (refer to Patent Document 1). In general, in a liquid crystal/cured material composite device, a nematic liquid crystal phase is uniformly dispersed in a cured material phase, the device is configured to control transparency-scattering of light by switching matching/mismatching of refractive indexes of the cured material phase and the liquid crystal phase by applying a voltage. This liquid crystal/cured material composite device requires no polarizer in principle, and accordingly, it has high light transmittance. For this reason, for example, such a device is suitable for applications such as light shutters to be used for e.g. a sunroof for an automobile, show windows or various types of bulletin boards capable of displaying characters or patterns, or instrument panels or windows of automobiles. As an example of such a liquid crystal/cured material composite device, a device showing a transparent state at a time of no voltage application, is also reported (refer to Patent Document 2).
However, most of the above liquid crystal/cured material composite devices are usually required to contain the cured material in an amount of usually at least 20 mass %, preferably at least 30 mass % (refer to Patent Documents 3 and 4). Here, since the liquid crystal phase shows a plurality of refractive indices while the cured material phase usually shows only one refractive index, there has been a problem that when the device is used for e.g. a large window glass, the haze value of the device in transparent state is high except in a direction where the refractive indexes agree with each other. Namely, there has been a problem that in the transparent state, when a panel is observed in a direction perpendicular to the panel, the panel looks transparent, but when the panel is observed in a slant direction, the panel looks not sufficiently transparent. Further, in a case of liquid crystal/cured material composite device produced by a polymerization-phase-separation method (a method of producing phase separation of liquid crystal and cured material by polymerizing a liquid crystal mixture containing more amount of monomer than liquid crystal, that is described in examples of Patent Documents 3 and 4), when a liquid crystal phase having high heat-resistant temperature, that is, high phase-transition temperature Tc is required, in order to prevent liquid crystal phase from segregating in a uniform liquid crystal mixture before polymerization, it is necessary to polymerize the liquid crystal mixture while it is heated. In order to solve the above two problems, a PSCT (Polymer Stabilized Cholesteric Texture) is disclosed (refer to Patent Document 5), which is produced by adding a small amount of curable compound to a chiral nematic liquid crystal having a helical pitch selectively reflecting visible light, to stabilize focal conic alignment of the chiral nematic liquid crystal so that the liquid crystal shows a scattering state at a time of no voltage application.
Patent Document 1: U.S. Pat. No. 4,688,900
Patent Document 2: JP-A-2000-119656
Patent Document 3: U.S. Pat. No. 4,834,509
Patent Document 4: U.S. Pat. No. 5,200,845
Patent Document 5: U.S. Pat. No. 5,437,811